Mixing systems

ABSTRACT

A system for providing improved bulk liquid mixing and effective gas-liquid contacting for mass transfer of the gas to the liquid, especially a non-Newtonian liquid, the viscosity of which decreases when under shearing conditions (shear thinning), in an upright tank. A process, such as fermentation which produces commercial quantities of polysaccharides such as xanthan gum, may be carried out in the tank. An upright draft tube is mounted within the tank and has a lower end spaced from the tank bottom and an upper end spaced below the surface of the liquid in the tank. A plurality of mixing impellers in the draft tube are sufficiently close to each other to establish a field or pattern of agitation to cause shear thinning and upflow throughout the draft tube and which may produce turbulence at the liquid surface. A plurality of radially inwardly projecting, circumferentially spaced baffles extend from the draft tube and are proximate the mixing impellers to prevent swirling of the liquid within the draft tube. Gas may be sparged into the vessel in or adjacent to the lower end of the draft tube. A circulating co-current flow of gas and liquid, with intimate gas-liquid contact, is induced up through the draft tube and out the upper end of the draft tube so as to provide a high rate of mass transfer from the gas to the liquid. The flow including entrained gas turns down through the annular region between the tank wall and the draft tube for recirculation of the liquid and gas above the surface of the liquid, resulting in high gas holdup and gas-liquid contact in the downflow annular region and in the upflow draft tube region, without gas flooding of the impellers. The concentration of dissolved gas and the effectiveness of the system for gas-phase mass transfer is determined by reaeration of a test sample of the liquid. Additional surface aeration can be provided with a surface impeller having a plurality of blades with vertical portions and outwardly inclined portions, the vertical portions being inclined with respect to radial lines from the axis of rotation of the impeller.

The present invention relates to mixer systems, and particularly tosystems (methods and apparatus) for the circulation and gas-liquidcontacting of liquids in a tank, especially when such liquids havenon-Newtonian, shear thinning viscosity characteristics. Goodcirculation and mixing of the liquid and intimate gas-liquid contactingfacilitates mass transfer of a gaseous component into the liquid.

The invention is especially suitable for use in bio-reaction processes,such as fermentation by circulating slurries containing microbes andgrowth media, especially where the fermentation process increases theviscosity of the slurry. The present invention enables improvedoxygenation and mixing of such liquids to promote the fermentationprocess. A fermentation process in which the invention finds particularapplication is a process for producing polysaccharides such as xanthangum, and improves such process by enabling increased circulation andmixing of the solution and oxygenation thereof at high concentrations ofxanthan gum which results in such high viscosities so as to precludeeffective circulation and mixing thereof by conventional means, suchthat the value of the product of the fermentation, which is a functionof the concentration of xanthan gum, is increased or produced in ashorter period of time.

Non-Newtonian liquids which can be effectively mixed and oxygenated witha mixing system embodying the invention have shear thinningcharacteristics, that is the viscosity of such liquids decreasessignificantly in the presence of shear. Regions in which shear isproduced, in the attempt to reduce the viscosity of the liquid so as toenable it to be circulated, have in conventional systems been confinedto the immediate vicinity of the impellers used to circulate the liquid.Such regions have sometimes been referred to as caverns of shear thinnedliquid surrounding the impellers. The remaining liquid, for example in atank in which the impellers are located, remains at high viscosity andthus does not circulate or mix to the extent required for effective gastransfer, and particularly oxygenation of the entire body of liquid inthe tank. These non-mixed or non-circulating portions of the tank liquidare often referred to as "dead zones" and significantly reduce theoverall effectiveness of the fermentation process.

It has been discovered in accordance with the invention that circulationof a substantial volume fraction of the liquid in the tank enablescirculation of the entire body of liquid in the tank. In the case ofshear thinning (non-Newtonian) liquids, a shear field or pattern ofagitation effects circulation of the liquid when it is achieved within asubstantial volume of the liquid in a tank. Shear fields or patternswhich produce flow in a direction other than the direction ofcirculation, for example, a swirling flow, is inhibited in accordancewith the invention. The flow through the substantial volume of liquidcauses flow elsewhere throughout the tank thereby circulating the entirebody of liquid to obtain good top to bottom turnover of the liquid inthe tank. The introduction of gas into the circulating flow and thegasification thereof as may be required by the process, for example afermentation process ongoing in the tank, is then achievable.

It has been proposed to use various expedients for enhancing mixing andcirculation in a tank. However, these techniques have been unable toprovide adequate circulation and mixing at flow rates sufficient tofacilitate the process under the severe circulation and mixingconditions such as presented by many non-Newtonian, shear thinningliquids, especially in fermentation processes.

Accordingly it is an object of the invention to provide an improvedmixing system which enables effective mixing and circulation of liquidsunder severe mixing conditions, especially those presented bynon-Newtonian (shear thinning) liquids.

It is a still further object of the present invention to provide animproved system involving circulation of liquids and the gasificationthereof which can be carried out effectively with high viscosity, shearthinning liquids.

It is a still further object of the present invention to provideimproved impeller systems, which effect circulation of liquids in atank, which are efficient in terms of the power required to produce arequired flow in the tank.

It is a still further object of the present invention to provide animproved impeller which facilitates surface gasification by creating aspray of liquid above the surface of the liquid in the tank, suchsurface gasification being referred to herein as surface aeration,without limitation to the nature of the gas (whether air or oxygen orsome other gas) at the surface of the liquid in the tank.

It is still a further object of the invention to provide a mixingenvironment that reduces the apparent viscosity of the solution andthereby increases the liquid phase mass transfer and thus increases theoverall gas-liquid mass transfer.

It is a still further object of the present invention to provide animproved method of determining the efficiency of gasification which isreferred to herein as mass transfer of the gas to the liquid in terms ofan overall liquid phase mass transfer coefficient, k_(L) a, andparticularly to a method for measurement of the oxygenation of theliquid by unsteady state reaeration so as to enable such measurements tobe accurately made where standard dissolved oxygen probes and standardWinkler dissolved oxygen titration procedures are not useful because ofthe high viscosity and ineffective mixing of the bulk liquid phase andthe opaqueness of the aerated liquid medium.

Briefly described, the present invention may be embodied in a mixersystem disposed below the surface of the liquid in a tank (the surfacebeing measured when the liquid is static, as when not being circulated)and utilizes a plurality of impellers spaced from each other along theaxis of a stationary draft tube, around which axis the impellers arerotated. The draft tube provides coaxial regions inside and outside ofthe tube, with the diameter of the tube and its length being such thatthe tube occupies a substantial volume fraction of the liquid in thetank. The impellers include a plurality of impellers, and the impellerscreate a shear field or pattern of agitation and a pressure gradient toproduce good circulation upwardly through the inside region and thendownwardly through the outside region. The impellers provide agitationfields which are coupled to each other, and particularly which overlap.Swirling flow inside the draft tube is inhibited, for example, bybaffles which project radially inwardly from the draft tube wall andaxially between the impellers and preferably above and below the upperand lower most impellers.

Gas may be sparged (injected) into the flow entering the draft tubeand/or at the liquid surface. In such event the gasification byentrainment of gas in the tank above the liquid surface may be enhancedby the use of a surface aeration impeller. Also the circulation at thetop of the draft tube may be enhanced by a shroud which bridges theinner and outer regions.

The surface aeration impeller may be provided by a plurality of bladesspaced circumferentially from each other and disposed at acute angles toradial lines from the axis of rotation of the impeller. The lowerportions of the blades, which may extend below the surface, may befolded outwardly. The blades drive the liquid into a spray umbrella in adirection upwardly and outwardly away from the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention as well as presently preferred embodiments thereof and thebest mode of carrying out the methods provided by the invention willbecome more apparent from the reading of the following description inconnection with the accompanying drawings in which:

FIG. 1 is a diagrammatic, sectional front elevational view of a tankcontaining a mixing impeller system in accordance with the invention;

FIG. 2 is a sectional view taken along the line 2--2 in FIG. 1 whenviewed in the direction of the arrows;

FIG. 3 is a view similar to FIG. 1 showing another embodiment of animpeller system in accordance with the invention;

FIG. 4 is a sectional view taken along the line 4--4 in FIG. 3;

FIG. 5 is a view similar to FIG. 1 showing another embodiment of theinvention;

FIG. 6 is a view, similar to FIG. 1, showing still another embodiment ofthe invention;

FIG. 7 is a sectional view taken along the line 7--7 in FIG. 6 whenviewed in the direction of the arrows;

FIG. 8 is a view, similar to FIG. 1 showing still another embodiment ofthe invention;

FIG. 9 is a view, similar to FIG. 1, showing still another embodiment ofthe invention:

FIG. 10 is a sectional view along the line 10--10 in FIG. 9 when viewedin the direction of the arrows;

FIG. 11 is a sectional view along the line 11--11 in FIG. 9; and

FIG. 12 is plot showing the changes in concentration of oxygen in a bodyof liquid, as a function of time, which has been oxygenated in a systemsuch as shown in FIG. 1, and which plot is useful in deriving the masstransfer coefficient, K_(L) a, resulting from oxygenation of the body ofliquid.

Referring first to FIGS. 1 and 2, there is shown a mixing system whichhas been found practical and effective for use in processes for mixingand circulating and gasifying non-Newtonian liquids and liquid slurries.The system of FIG. 1 has been found especially suitable for improvingthe mixing and mass transfer rate in bioreaction processes, includingfermentation, and particularly a process for producing xanthan gumsolutions. Presented hereinafter are examples illustrating a mixing andoxygenation process using air as the gaseous oxygenation medium on asolution simulating a high concentration (three to four percent byweight) solution of xanthan gum. The ability to effectively mix andoxygenate such solutions and produce the products of fermentation atconcentrations having enhanced commercial value is an important featureof this invention.

Non-Newtonian liquids are characterized by having a variable viscositywhich is a function of the applied shear force. Newtonian liquids suchas water and mineral oil have a constant viscosity. The viscosity ofnon-Newtonian liquids changes when in a shear field, that is where theliquid is subject to a changing shear rate (1/sec). Xanthan gumsolutions in the three percent by weight range are a commerciallyrelevant non-Newtonian solution in that their viscosity, when at rest,is of the order of 10,000 cp, (centipoise) (for example 10,000 to 30,000cp) and 100 cp when subject to a high shear rate. All viscosity valuesreferred to herein are determined on a Brookfield viscometer.

In gasification processes, such as the oxygenation of the broth infermentation, it is desirable to continue gasification until the oxygencan no longer be transferred at a high enough rate to sustain themicroorganisms in the fermentation broth. When approaching thiscondition in polysaccharride fermentations, the viscosity increases to apoint that the rate of oxygenation and circulation of the broth can nolonger supply the oxygen demand of the microorganisms. When thiscondition is reached, the fermentation cannot be continued. The mixingsystems provided by the present invention maintain non-Newtonianliquids, such as xanthan gum solutions, at low viscosity by maintaininga large fraction of the solution under high shear rates, even underthese conditions (high three percent or greater xanthan gumconcentrations) so that continued effective oxygen transfer and mixingcan be maintained. Thus, as the process proceeds from startup, the lowviscosity initial condition (100 to 200 cp in xanthan gum solutions) ismaintained because of the high shear rate in a significant volumefraction of the liquid being mixed and circulated. Gasification can thenproceed to higher xanthan gum concentrations in the liquid a largefraction of which is maintained at sufficiently low viscosity to enablegood circulation and oxygen transfer throughout the entire volume ofliquid in the fermenter.

As shown in FIGS. 1 and 2, the liquid is in a tank 10 and has a liquidlevel 12 below the upper end or rim 14 of the tank when the liquid inthe tank is static (that is not being circulated or turned over) betweenthe surface 12 and the bottom 16 of the tank. The tank 10 may begenerally cylindrical and the tank walls arranged vertically upright. Acylindrical draft tube 20 is mounted preferably centrally of the tank.Then the axis of the draft tube 20 is coincident with the axis of thetank 10 when the tank is cylindrical. The diameter of the draft tube andits length is such that the internal volume defined by the tube 20 is asubstantial part, at least 25% and preferably 50% of the volume of theliquid in the tank 10. There is clearance between the bottom 16 of thetank and the lower end 22 of the draft tube. The upper end 24 of thedraft tube is in the vicinity of the static liquid surface 12. Aplurality of mixing impellers 26, 28, 30 and 32 are attached to, anddriven by, a common shaft 34. The upper end of the shaft may beconnected to a drive motor via a gear box (not shown) and the lower endof the shaft 34, may be journaled in a steady bearing 36. The impellersare all of the same type, namely so-called pitched blade turbines (PBT),having a plurality of four blades circumferentially spaced about theaxis of rotation, the axis being the axis of the shaft 34 and the bladesare disposed at 45° to that axis. Such PBT impellers are available fromthe Lightnin Unit of General Signal Corporation, Rochester, N.Y. 14611,USA, as their Model A200. Alternatively, other axial flow impellers maybe used, such as airfoil-type blades (sometimes called hydrofoilblades). Such air foil axial flow impellers may, for example, be theA-315, which is presently available from the Lightnin Unit and which isdescribed in Weetman, U.S. Pat. No. 4,896,971. Other air foil impellerswhich may be suitable are described in U.S. Pat. No. 4,468,130, alsoissued to Weetman.

The mixing system in the draft tube also includes sets 38, 40, 42 and 44of four vertical baffles which are 90° displaced circumferentially aboutthe axis of the shaft 34, as shown in other FIGS. discussed hereinafter,between the impellers. Other sets of baffles may be located above, and,if desired below, the upper and lower most impellers 32 and 26. In otherwords, two pairs of baffles are contained in each set and the pairs are180° displaced with respect to each other (See FIG. 2). The impellers26-32, with the aide of the sets of baffles 38-44, produce a field orpattern of agitation which provide a high level of shear in the liquidin the draft tube. Thus, in the case of non-Newtonian, shear thinningliquids, the viscosity of the liquid in the draft tube is maintainedsufficiently low so that it enhances mass transfer and promotes improvedcirculation in the tank. The circulation, which has been found toproduce the most effective mixing, is in the upward direction inside thedraft tube to regions at the ends of the draft tube 32 and 24 where theflow changes direction, so that the flow is downward in the annularregion between the draft tube 20 and the sidewall of the tank 10.

The annular region between the draft tube wall 20 and the sidewall ofthe tank 10 is a region of low shear and hence high effective viscosityfor shear thinning liquids. Nonetheless, good uniform flow with nostagnant regions is maintained down through this high viscosity annularregion by virtue of the high flow rate generated up through the lowviscosity draft tube zone. Thus, the annular region between the drafttube wall 20 and the sidewall of the tank 10 has a relatively highaverage axial fluid velocity and the liquid is quickly recirculated intothe high shear, low viscosity draft tube region.

The relative sizing of the draft tube diameter and impellers and theirlocations in the draft tube are related by the flow rate so that therequisite circulation and mixing may be obtained. Then the rate of flowand volume contained in the draft tube and the volume of the tube aresufficient to establish the axial flow between the tube and wall of thetank over a broad range of viscosities up to and including viscositiesof the order of 10⁴ cp (Brookfield).

To prevent stagnant zones at the comer formed by the sidewall and thebottom 16 of the tank 10, it is desirable to install an annular plate orring 46 which defines a fillet to smooth the flow past the comer.Alternatively, the plate may be convexly, inwardly curved so as toprovide a generally circular contour for the fillet 46. In order togasify the liquid, a sparge pipe 50 directs the gas into the lower endof the draft tube, preferably in proximity to the tips (the radiallyoutward most or peripheral ends) of the blades of the lower mostimpeller 26. The introduction of the gas is known as sparging. The termaeration is generally used to connote the introduction of any gasincluding atmospheric air or oxygen enriched air. Substantially pure (90to 95%) oxygen may also be used. Gas dispersion or gas incorporationinto the liquid also occurs due to turbulence at the liquid surface 12where there is gas-liquid contacting and entrainment of the gas into theliquid so that it recirculates downwardly through the outer annularregion. Because of the high shear rate in the draft tube, the liquid isat low viscosity and enables the gas from the sparge pipe 50 to bebroken up into fine bubbles which present a large total gas-liquidinterfacial area to facilitate mass transfer. The effectiveness ofoxygen mass transfer may be measured in terms of the overall liquidphase mass transfer coefficient (K_(L) a).

In order to provide the high shear conditions (high shear ratesufficient to reduce the viscosity of the liquid in the tank so that itcan circulate readily and uniformly), the impellers 26, 28, 30 and 32are spaced sufficiently close to each other so that the field or patternof their flow overlaps. When the overlapping fields of flow is created,the agitation produces not only axial, but also significant radial forceon the fluid. The sets 38, 40, 42 and 44 of baffles inhibit this radialcomponent, which produces a swirling flow, so that the flow upwardthrough the draft tube is substantially axial. The baffles preferablyproject radially inwardly by distances sufficient to inhibit the radialflow of the liquid. Preferably, the height of the baffles is such thatthe spacing between the upper and lower edges of the baffles and theadjoining impellers is the minimum to provide a practical runningclearance for the impellers 26-32.

The following parameters have been found to provide suitable conditionsfor effective liquid circulation and mixing and mass transfer andoxygenation. It will be appreciated that the specific values which areselected, depend upon the material (liquid, liquid slurry or othermedium) being circulated and aerated. The characteristics are generallylisted in their order of criticality. It is a feature of the inventionto provide a mixing system wherein each of these parameters is used soas to secure the benefits of efficient liquid mixing and circulation andeffective gas-liquid contacting (mass transfer), especially inbio-reaction processes. The parameters are as follows:

1. The ratio of the draft tube diameter to the tank diameter is betweenabout 0.35 and 0.75, with a ratio of about 2/3 (.667) being presentlypreferred.

2. The ratio of impeller diameter to draft tube diameter is from about0.5 to 0.96. All of the impellers 26-32 are generally of the samediameter between the tips of the blades. If impellers of differentdiameter are used, the largest diameter impeller is used in selectingthis parameter, i.e. the ratio of the impeller diameter to the drafttube diameter.

3. Impeller vertical spacing, that is the distance between the meanheight of the impeller, as measured between the leading and trailingedges of the blades thereof, is between 0.70 and 1.30 of the diameter ofthe largest of two adjacent impellers. In other words, where theadjacent impellers have the same diameter, they may be from about 0.70to 1.30 of an impeller diameter apart. Where the adjacent impellers havedifferent diameters, the largest diameter is used to determine spacingin the range 0.70 to 1.30 of the larger of the two adjacent impellerdiameters. Preferably, the impellers are spaced apart so that theirmidlines are separate by about 1.0 impeller diameter.

4. The ratio of the radial width of the vertical baffles inside thedraft tube to the diameter of the draft tube is preferably in the rangeof about 0.1 to 0.4. A ratio of about 0.33 of radial width to draft tubediameter is presently preferred. The height of the baffles in thevertical direction should approach the impellers, and preferably beadjacent thereto, allowing only sufficient spacing for rotation of theimpellers without interference.

5. There preferably are two to four baffles in each set of bafflesadjoining the impellers.

6. The upper end of the draft tube may be submerged from the liquidsurface up to about 0.3 of the diameter of the draft tube. In caseswhere a surface aeration impeller is used or where a diverting shroud isused, as will be described hereinafter in connection with FIGS. 8 to 11,the submergence of the draft tube may be sufficient to enable insertionof the surface aerator and/or the flow diverter at the top of the drafttube. However, the volume of the liquid in the tank occupied by thedraft tube should remain substantial and be at least about 0.25 of thevolume of the liquid in the tank (between the bottom of the tank, theliquid level and within the sidewalls of the tank). The uppermostimpeller should also be less than about one impeller diameter from thesurface of the liquid in the tank. The placement of the uppermostimpeller is selected which engenders good surface turbulence and furthergas-liquid contacting for enhancing the gas transfer rate and the masstransfer coefficient of the system.

7. The off-bottom clearance of the bottom of the draft tube ispreferably from about 0.3 to about 0.7 of the draft tube diameter. Thepreferred parameter is 0.5 of the draft tube diameter for the spacing oroff-bottom clearance of the bottom or lower end of the draft tube fromthe bottom of the tank.

Referring to FIGS. 3 and 4, there is shown an impeller system havingfour impellers 60, 62, 64 and 66. There are two axial flow impellers 60and 64 and two radial flow impellers 62 and 66 which are disposedalternately along the axis of rotation (which is the axis of a shaft 68which is common to all of the impellers). The axial flow impellers maybe PBT's or airfoil blade impellers, such as discussed in connectionwith FIGS. 1 and 2. The radial flow impellers 62 and 66 may be so-calledRushton turbines, such as R-100 class radial flow impellers, which arepresently available from the Lightnin Unit of General SignalCorporation. Information as to the design of radial flow impellers maybe found in Englebrecht and Weetman, U.S. Pat. No. 4,454,078 andStanton, U.S. Pat. No. 4,207,275.

FIG. 3 also illustrates a lowermost set 70 of vertical baffles which mayextend outwardly from the lower end 22 of the draft tube, or the loweredge of the baffle may be coincident with the lower end of the drafttube.

Gas is sparged into the lower end of the draft tube. The radial andaxial flow impellers are closely coupled so that their agitationpatterns and shear fields overlap thereby enabling good axial upwardcirculation of the liquid through the draft tube and recirculationthrough the annulus between the draft tube and the sidewall of the tank.

The system shown in FIG. 5 is similar to the system shown in FIG. 3except that the draft tube occupies a larger volume fraction of theliquid in the tank and the baffles extend radially inward a lesserdistance in respect to the diameter of the larger impellers (the axialflow impellers 60 and 64 ) than in the case of the system shown in FIGS.3 and 4.

The use of alternate axial and radial flow impellers provides foradequate mixing, circulation and gasification, and affords amplecirculation rates (for example 1/2 foot per second flow), even in theannulus around the draft tubes, so as to produce good liquid mixing andtop to bottom turnover and ample mass transfer of the gas to the liquidand dispersion and solution of the gas into the liquid. To the extentthat the parts shown in FIGS. 3, 4, and 5 are similar to those shown inFIGS. 1 and 2, like reference numerals are used.

Referring to FIGS. 6 and 7, there is shown a mixing system 80 in thetank 10 having three axial flow impellers 82, 84 and 86 inside the drafttube 20 and a radial flow impeller 88 on the shaft 34 common to all theimpellers. The radial flow impeller 88 is located below the draft tubein the region where the liquid flow turns upwardly into the draft tube.In the case of the impellers in the draft tube, the baffles, especiallyin the set of baffles at the uppermost and lowermost end of the drafttubes (sets 90 and 92 in FIG. 6), reduces swirl, as well as promotingthe circulation upwardly through the draft tube and then down into theannulus between the sidewalls of the tank and the draft tube 20. Theradial flow impeller has a power number (the ratio of the power in horsepower which is needed to drive the impeller to the product of the cubeof the speed of the impeller and the diameter to the fifth power of theimpeller) which is much higher than and preferably about equal to thesum of power numbers of the impellers in the draft tube; thus the radialflow impeller 88 draws at least as much power as all of the threeimpellers 82, 84 and 86 in the draft tube. The agitation field from theradial flow impeller 88 extends up to the lower end of the draft tubeand facilitates the creation of the agitation pattern and shear field ina sufficient volume of the tank to promote complete turnover orcirculation (top to bottom mixing) of the liquid in the tank 10. Alsothe radial flow impeller facilitates effective dispersion of the gasfrom the sparge pipe into the mixing system. To inhibit swirl in themixing pattern of the radial flow impeller 88, a plurality (at least twopairs) of vertical baffles 96 and 98 are disposed along the sidewalls ofthe tank 10 and extend at least half the distance from the bottom of thetank to the bottom of the draft tube for typical liquid media which arebeing mixed, circulated and aerated.

In the event that gasification is done with a gas other than air andespecially in all fermentation processes, it is desirable that theoverhead gas space (the distance between the rim 14 of the tank and theliquid level 12, shown at 100 in FIG. 1) be sealed by a cover 102. Forexample, when oxygen is used as the gas for aeration purposes, theoverhead is desirably sealed. The oxygen may be introduced by a conduitwhich enters the overhead 100 via the sidewall of the tank.

Referring to FIG. 8, there is shown another embodiment of the inventionutilizing a mixing system 200, having a radial flow surface aerationimpeller 202 with blades circumferentially spaced about the axis ofrotation 204 of the common drive shaft 206 of the impeller 202 and fouraxial flow 45° PBT impellers 208, 210, 212 and 214. The surface aerationimpeller 202 may suitably be a Lightnin Model R-335 impeller andprovides additional gas-liquid interfacial area by affording a sprayumbrella, thereby entraining additional air back into the surface liquidwhich is recirculated down the angular region between the draft tube 220and the sidewalls of the tank 222. The surface aerator 202 also providesadditional upward liquid pumping action through the draft tube 220.Where a surface aerator is used, the upper end 224 of the draft tube maybe coincident with the surface 226 of the liquid(the static liquid levelin the tank).

In the system shown in FIG. 8, separate sparge pipes 228 are providedwhich extend downwardly along the side wall of the tank to the lowermostset 230 of baffles. The sparge pipes hook upwardly into the draft tubeso as to facilitate introduction of the gas directly into the drafttube. It is a feature of the invention to provide sparging either intothe draft tube or into the overhead gas space or both. Whensubstantially pure oxygen is used, it is introduced into the overheadspace, into the sparge pipes, such as the array of pipes 228, or bothinto the overhead and into the array of pipes.

Referring to FIGS. 9, 10 and 11, there is shown an impeller system 300having, in addition to an arrangement of impellers and baffles in adraft tube 302, similar to the arrangement of impellers 208, 210, 212and 214 and their associated baffles, a shroud 304 to facilitate thediversion of the upward flow of liquid out of the draft tube into theannular region between the draft tube 302 and the sidewalls of the tank306. This shroud may be a hemi-toroidal shell which has radial bafflesat least one and preferably two pairs of baffles 310, 180° apart areused to inhibit radial flow and short circuiting of the flow back intothe draft tube. These baffles extend radially inwardly fromapproximately the draft tube to the inner periphery of the shroud 304.The shroud 304 may be stiffened by a rod or angle iron 312. Theconnection to the draft tube may be by welds along the lower edges ofthe baffles 310 and the upper edge of the draft tube.

Extending through the static surface level 316 of the liquid in the tank306, is an improved surface aeration impeller. This impeller has aplurality of vertically extending blades 320. Each blade is disposed atan angle (alpha--∝) of approximately 30° to a successive,circumferentially spaced radial line around the axis of the impeller(the axis of the common shaft 322). These blades have vertical portions324 at the upward ends thereof. The blades 320 also have, preferablyextending below the liquid surface 316, portions 326 which are bentoutwardly and define obtuse angles of approximately 120 to 135 degreeswith respect to the vertical portions 324 thereof. The blades act asscoops to provide ample flow to the spray umbrella liquid from thesurface aerator. The spray falls back over the shroud 304 into theannular region between the sidewalls of the tank 306 and the draft tube302, thereby further facilitating the entrainment of gas from the headspace above the liquid and providing a larger mass transfer coefficient,K_(L) a. The embodiment shown in FIGS. 9-11 may be preferable when highpurity oxygen is used as the gas in the process carried out in the tank306.

Referring to FIG. 12, there is shown a curve of dissolved oxygen, D.O.,concentration measurements with time which is useful in determining theoverall liquid phase mass transfer coefficient, K_(L) a. This masstransfer coefficient may be determined by means of an unsteady statereaeration test which uses dissolved oxygen concentration measurementswith time from dissolved oxygen probes in the solution or by directliquid sample titration. However, accurately determining the D.O.concentration measurements with time for high viscosity, opaque mediasuch as xanthan gum solutions is extremely difficult. The accurate useof the unsteady state reaeration test procedure also requires goodliquid mixing in the overall bulk liquid phase with no dead zones. Alsoregions of high flow are required where accurate calibration and use ofD.O. probes can be achieved. The present invention provides an effectiveliquid mixing and circulation system which satisfies all of theserequirements for accurate use of the unsteady state reaeration testprocedure. Conventional liquid mixing systems cannot be effectivelyevaluated using the unsteady state test procedure for high viscosity,shear thinning fluids because of the poor level of bulk liquid mixingand turnover in the tank.

The unsteady state reaeration test is carried out by first making up abatch of the xanthan gum solution by mixing and aerating in the actualmixing and aeration system under test. This may be done for severalhours so that there is assurance that the equilibrium dissolved oxygenlevel has been reached. Then, a test sample is extracted from the batchand the equilibrium dissolved oxygen content is measured using amodified Winkler dissolved oxygen titration procedure specificallyadapted for high viscosity opaque solutions. The dissolved oxygencontent at saturation is then used to calibrate the dissolved oxygenprobes for the equilibrium-dissolved oxygen level (milligrams per literof dissolved oxygen) in solution.

After the above D.O. probe calibration procedure is completed, the tankliquid is stripped of dissolved oxygen by bubbling a non-reacting gas,for example, nitrogen, through the batch in the tank. This may be donein the mixing system by replacing air or oxygen as the aerating gas withnitrogen. Measurements were made with the dissolved oxygen probe to showthat the dissolved oxygen has been stripped from the liquid solution.This stripping may take 10 to 15 minutes. Then, reaeration with air orother oxygen containing gas is carried out until oxygen saturation isreached in the bulk liquid phase. Measurements are made of D.O.concentration, during reaeration, at successive periods of time. Thenthe curve, FIG. 12 is plotted. t_(o) is the start of reaeration andC_(o) is the initial D.O. concentration.

The oxygen transfer rate (OTR) at any point in time is the slope of thecurve in FIG. 12 or dC/dt. The slope of the curve is also defined asbeing equal to K_(L) a (C*-C), where C* is equal to the equilibrium D.O.level, for example, from a sample taken from the middle of the tank. Thesolution of the resulting differential equation is equal toC=C*-(C*-C_(o)) exp [-(K_(L) a)]. A statistical solution of the equationfor the D.O. concentration versus time profile provides an overalllumped parameter K_(L) a for the oxygen mass transfer process. This masstransfer coefficient, K_(L) a, is a measure of the effectiveness of theaeration in the mixing process and is used in the examples presentedbelow to demonstrate the effectiveness of the process for differentprocess conditions and parameters.

In examples 1 through 5, a system such as shown in FIGS. 1 and 2 is usedwhere the impellers are 17 inch diameter PBT's. A four pipe gas spargesystem such as shown in FIG. 8, rather than a single pipe sparge 50, wasused. The liquid which was tested in the examples was a solutionsimulating a xanthan gum fermentation broth containing three to fourpercent by weight xanthan gum. The simulating solution was a solution oftwo percent by weight xanthan gum and 0.5M(molar) sodium sulfate, inwater. The mass transfer coefficients given in the examples as theoverall tank volume liquid phase mass transfer coefficient were measuredusing the unsteady state reaeration technique as specifically developedfor directly and accurately measuring the liquid phase mass transfercoefficient for xanthan gum solutions as discussed above.

EXAMPLE 1

    ______________________________________                                        Draft Tube Diameter     18"                                                   Tank Diameter           36"                                                   Liquid Level            72"                                                   Tank Height             84"                                                   Power Input             23.6 HP/kgal                                          Gas sparge rate         0.5 vvm                                               Liquid flow rate up through                                                                           3/2 liters/sec.                                       draft tube and down through                                                   annular region.                                                               Overall tank liquid     3.9 sec                                               turnover time                                                                 Overall tank volume liquid phase                                                                      18.9 sec.sup.-1                                       mass transfer coefficient (k.sub.L a)                                         ______________________________________                                    

EXAMPLE 2

    ______________________________________                                        Draft Tube Diameter     24"                                                   Tank Diameter           36"                                                   Liquid Level            72"                                                   Tank Height             84"                                                   Power Input             23.6 HP/kgal                                          Gas sparge rate         0.5 vvm                                               Liquid flow rate up through                                                                           5/6 liters/sec.                                       draft tube and down through                                                   annular region.                                                               Overall tank liquid     2.3 sec                                               turnover time                                                                 Overall tank volume liquid phase                                                                      16.5 hr.sup.-1                                        mass transfer coefficient (k.sub.L a)                                         ______________________________________                                    

EXAMPLE 3

    ______________________________________                                        Draft Tube Diameter    18"                                                    Tank Diameter          36"                                                    Liquid Level           72"                                                    Tank Height            84"                                                    Power Input            23.6 HP/kgal                                           Gas sparge rate        0.1 vvm                                                Liquid flow rate up through                                                                          444 liters/sec.                                        draft tube and down through                                                   annular region.                                                               Overall tank liquid    2.7 sec                                                turnover time                                                                 Overall tank volume liquid phase                                                                     10.3 hr.sup.-1                                         mass transfer coefficient (k.sup.L a)                                         ______________________________________                                    

EXAMPLE 4

    ______________________________________                                        Draft Tube Diameter    24"                                                    Tank Diameter          36"                                                    Liquid Level           72"                                                    Tank Height            84"                                                    Power Input            23.6 HP/kgal                                           Gas sparge rate        0.1 vvm                                                Liquid flow rate up through                                                                          456 liters/sec.                                        draft tube and down through                                                   annular region.                                                               Overall tank liquid    2.3 sec                                                turnover time                                                                 Overall tank volume liquid phase                                                                     9.2 hr.sup.-1                                          mass transfer coefficient (k.sub.L a)                                         ______________________________________                                    

EXAMPLE 5

    ______________________________________                                        Draft Tube Diameter    18"                                                    Tank Diameter          36"                                                    Liquid Level           72"                                                    Tank Height            84"                                                    Power Input            15.75 HP/kgal                                          Gas sparge rate        0.5 vvm                                                Liquid flow rate up through                                                                          240 liters/sec.                                        draft tube and down through                                                   annular region.                                                               Overall tank liquid    5.1 sec                                                turnover time                                                                 Overall tank volume liquid phase                                                                     16.5 hr.sup.-1                                         mass transfer coefficient (k.sub.L a)                                         ______________________________________                                    

EXAMPLE 6

    ______________________________________                                        Draft Tube Diameter    24"                                                    Tank Diameter          36"                                                    Liquid Level           72"                                                    Power Input            15.75 HP/kgal                                          Gas sparge rate        0.5 vvm                                                Liquid flow rate up through                                                                          276 liters/sec.                                        draft tube and down through                                                   annular region.                                                               Overall tank liquid    4.4 sec                                                turnover time                                                                 Overall mass transfer coefficient (k.sub.L a)                                                        12.5 hr.sup.-1                                         ______________________________________                                    

In addition to the performance data included in the above examples, thenew mixer system designs have no dead zones anywhere within the entiretank system bulk liquid phase and also achieve very effective gasdispersion throughout the tank. The average bubble size escaping fromthe liquid surface is in the range of 1/4" to 1/2" in diameter ascompared to 8" to 12" for conventional design systems. Also themechanical stability of the entire mixer and tank system is greatlyimproved with essentially no violation or erratic movement of the mixerand tank system.

From the foregoing description, it will be apparent that there has beenprovided improved mixer systems which are especially adapted forproviding effective liquid mixing and gas-liquid contacting and improvedmass transfer for non-Newtonian, shear thinning solutions. Variationsand modifications in the herein described systems, within the scope ofthe invention, will undoubtedly suggest themselves to those skilled inthe art. Accordingly, the foregoing description should be taken asillustrative and not in a limiting sense.

We claim:
 1. The method of providing bio-reaction in a tank whichcomprises the steps of introducing a slurry of a bacteria containingliquid and a growth medium into the tank, which slurry increases inviscosity as the bio-reaction proceeds, circulating said slurry upwardlyand downwardly in inner and outer coaxial regions of said tank, saidinner region occupying at least about twenty-five percent of the volumeof said tank which contains said slurry, with the aide of a draft tubewhich separates said regions, said circulating step including the stepof shearing said liquid throughout said inner region, and said shearingstep comprising producing axial flow in said inner region whileinhibiting swirl therein.
 2. The method according to claim 1 furthercomprising the step of introducing gas selected from the groupconsisting of air, oxygen enriched air and substantially pure oxygeninto said tank, said introducing step being carried out by selfgasification, sparging of said gas into said inner region or both bysparging and self gasification.
 3. The method according to claim 1further comprising the step of spraying said liquid at said surfacebetween said inner and outer regions with the aide of a surface aerationimpeller located at the surface above said inner region.
 4. The methodaccording to claim 1 further comprising the step of producing radialflow of said liquid between the bottom of said draft tube and the bottomof said tank and sparging said gas into said radial flow.
 5. The methodaccording to claim 1 further comprising the step of impelling saidliquid in said inner region radially and axially in alternate successiveportions of said inner region with the aide of impellers which produceprincipally radial flow in said alternate successive regions.